Explorers find unexpected company in the vast silence of Rama's interior.

There was an intruder in the camp.

Laura Ernst noticed it first. She froze in sudden shock,
then said: 'Don't move, Bill. Now look slowly to the
right.'

Norton turned his head. Ten metres away was a
slender-legged tripod surmounted by a spherical body no
larger than a football. Set around the body were three
large, expressionless eyes, apparently giving 360 degrees
of vision, and trailing beneath it were three whiplike
tendrils. The creature was not quite as tall as a man, and
looked far too fragile to be dangerous, but that did not
excuse their carelessness in letting it sneak up on them
unawares. It reminded Norton of nothing so much as a
three-legged spider, or daddy-long-legs, and he wondered
how it had solved the problem - never challenged by any
creature on Earth - of tripedal locomotion.

After regarding them passively for several minutes, the creature suddenly moved, and now they could understand why they had failed to observe its arrival. It was fast, and it covered the ground with such an extraordinary spinning motion that the human eye and mind had real difficulty in following it.

This quote illustrates how it seemed to accomplish its movements:

...each leg in turn acted as a pivot around which the creature whirled its body... it also seemed to him that every few 'steps' it reversed its direction of spin, while the three whips flickered over the ground like lightning as it moved.

It turns out that the 'spiders' are what the author calls 'biological robots' that were designed by the creators of Rama. Clarke coined the word "biot" to describe them. The spiders have 'considerable quantities of light metals.' The spiders have no mouth, no stomach, no gut, no lungs, no circulatory system. So, how does it move?

"Eighty percent of the body consist of a honeycomb of large cells... it's the one Raman structure that does exist on earth - though only in a handful of marine animals.

"Most of the spider is simply a battery, much like that found in electric eels and rays... It's the creature's source of energy."

SF fans will also, of course, remember the great tripods from H.G. Wells' 1898 classic War of the Worlds.